1. Field of the Invention
This invention relates to an improved process for the production of urea from ammonia and carbon dioxide via synthesis at adequate pressure and temperature. The urea formation takes place in a synthesis zone (or zones) where an excess of free ammonia is kept to favour high conversions.
This improved process covers, in particular, a new treatment step to recover and recycle the unreacted materials (free ammonia and carbamate) from the reaction zone (or zones) in an optimal way to minimize energy consumption and investment costs.
2. Description of the Prior Art.
It is known that high reaction yields are favoured by a high ammonia excess (compared with the stoichiometric ratio) which require however a high reactor operating pressure and, as a consequence, complex and energy consuming treatment sections downstream the reactor to remove and recycle said excess ammonia and the residual carbamate from the produced urea.
Some processes have been recently studied to minimize energy and investment requirement for the treatment sections downstream the reactor, but they are still complex and still require considerable amount of energy.
The U.S. Pat. No. 4,208,347 (Montedison), known as the IDR process (Isobaric Double Recycle), describes a two steps stripping treatment scheme where carbamate is removed with ammonia as stripping agent, in the first step, while free ammonia is removed with carbon dioxide as stripping agent, in the second step. A certain complexity of this scheme is evident. The U.S. Pat. No. 4,321,410 (Mitsui Toatsu Chemicals and Toyo Engineering) known as the ACES process (Advanced Process for Cost and Energy Saving) describes a two steps stripping treatment performed in a newly designed stripper where the urea reactor effluent is contacted with the gases (mainly NH.sub.3 and CO.sub.2) coming from a falling film exchanger in an adiabatic first treatment step where free ammonia is removed and successively treated in the falling film exchanger (second treatment step) counter-currently with carbon dioxide introduced as stripping agent to remove the residual carbamate.
With this process the amount of free ammonia that can be removed from the reactor effluent is limited due to the presence of NH.sub.3 in the gases contacting the urea solution in the adiabatic step, while a minimum content of free ammonia in the urea solution is desirable to obtain optimal carbamate removal in the subsequent CO.sub.2 stripping step.
The Italian patent application No. 24357A/80 (Snamprogetti) describes a process very similar to the Montedison one but with the two treatment steps at different pressure (non isobaric).
None of the above mentioned new processes achieve the direct recycle to the reactor of the free ammonia separated form the reactor effluent, which is optimal to minimize investment and energy consumption. The indirect recycle of ammonia in the downstream sections is made via acqueous solutions with the recycle of water in the reactor, which is detrimental for reaction yields.
The last generation processes, followed by the cited new generation ones, were dominated by the Stamicarbon CO.sub.2 stripping and Snamprogetti NH.sub.3 stripping processes both using only one high pressure treatment step. In the Stamicarbon CO.sub.2 stripping process, the reactor effluent with a low free ammonia content is directly treated in the CO.sub.2 stripper to remove the residual carbamate. The content of ammonia in the reactor is kept low to have optimal carbamate separation in the CO.sub.2 stripper, but reaction yields are low with consequent high investment and energy consumption.
In the Snamprogetti NH.sub.3 stripping process the reactor effluent with a higher free ammonia content is also directly treated in a "self stripping" treatment step to remove carbamate.
An important amount of free ammonia is still present in the urea solution leaving the stripper and is separately recycled to the reactor using pumps.
This scheme implies the use of a rectifying column to separate pure ammonia with high costs and energy consumption.
None of the last generation processes also achieves the direct recycle to the reactor of the free ammonia separated from the reactor effluents with minimum investment and energy consumption.